[R] how to delete some columns from a matrix based on some other indicator variable

[Ya-Hsiu Chuang]

Hello,

I am not very familiar with R and need help in deleting a few columns in a 
matrix.

Suppose I have a indicator variable called r and it's defined as r = (0, 0, 
1, 1). A matrix D is a 3X4 matrix. If I want a new matrix which contains 
only the columns of D corresponding to the elements of r that equal to 1. 
how can i write a loop which creat a new matrix that contains only the last 
2 columns of D in this case? thanks